1. Field of the Invention
This invention relates to cores for winding webs of deformable materials.
2. Description of the Prior Art
Published German Patent Application (Offenlegungsschrift) No. 3,610,557 describes cores for winding webs of paper. That specification describes the known problem that if the core is rigid, the adhesive tape which is used to secure the leading edge of the web of paper to the core will cause, by virtue of the finite thickness of the adhesive tape, an embossing of the paper for many turns of the paper on the core. This embossing occurs by virtue of the high radial pressure which builds up as successive turns are wound on the core. The leading edge of the web also causes such embossings. Paper which contains such embossings is, for many purposes, useless, and hence the end portion of the web, perhaps as much as several hundred feet, has to be discarded.
Offenlegungsschrift No. 3,610,557 describes a solution to the problem of embossings caused by the adhesive tape and the end edge of the web, and like irregularities. That described solution is to provide the core with a coating of elastically or plastically deformable material which deforms to accommodate the irregularity so that the first turns of the web on the core do not have to deform to accommodate the irregularity.
In the manufacture of webs of base material for photographic film, the problems which derive from lack of uniformity in thickness (often called "gage") across the web, are well known. One such problem derived from gage non-uniformity, is known as gage bands. If the region of increased thickness is at a constant position, considered laterally of the web, when the web is wound on a core, the increased thickness region of each turn will lie on top of the increased thickness region off the previous turn and what is known to those skilled in the art as a gage band, results. In the gage band there is localized very high pressure which often results in undesirable effects such as abrasion, deformation and chemical and/or physical changes. To overcome the problems of gage bands, it is known to knurl the margins of the web of base material so that the protuberances produced by the knurling are higher than any gage increase likely to be encountered in normal manufacturing. When the web with the knurling along its two margins is wound on a conventional rigid core having a non-flexible surface, the knurlings in the two margins wind on top of themselves and it is in those areas, rather than where the gage increases overlie one another, that the high pressure between adjacent turns is encountered. Prior to use in the production of film, the margins containing the knurlings are slit off and discarded and it may be safely assumed that the entire portion of the web between the margins is free from any defects attributable to gage bands.
It has been found that when a web of photographic base material, having knurlings along its margins, is wound on a core such as is described in Offenlegungsschrift No. 3,610,557, if the coating on the core is soft enough to avoid undesirable embossings in the web due to the end edge or the securing tape, the very high pressures progressively created by the knurlings stacked upon themselves cause the wound web to collapse radially inwards. Such collapses are, inevitably, not localized, considered in the direction axially of the roll, but extend along the roll axially from the margins towards the middle of the width of the roll. The collapses cause permanent damage to the web and, hence, the width of web at the margins which has to be slit off and discarded, is increased. This results in undesirably increased waste and correspondingly lower productivity.